Multilayer pigments having alternating layers of a material of high refractive index and a material of low refractive index are known. They comprise predominantly metal oxides. However, the material of high refractive index can also be replaced by a semitransparent metal layer. The metal oxide layers are produced either by a wet method, by precipitating the metal oxide hydrates from a metal salt solution onto a substrate material, or by vapour deposition or sputtering in a vacuum. For instance, U.S. Pat. No. 4,434,010 describes a multilayer interference pigment consisting of a central layer of a reflective material (aluminium) and alternating layers of two transparent, dielectric materials of high and low refractive index, for example titanium dioxide and silicon dioxide, either side of the central aluminium layer. In a further embodiment of the pigment the layers following the central aluminium layer are formed by magnesium fluoride and chromium. This pigment is employed for the printing of securities.
JP H7-759 (A) describes a multilayer interference pigment with a metallic lustre. It consists of a substrate which is coated with alternating layers of titanium dioxide and silicon dioxide. The substrate is formed from platelets of aluminium, gold or silver, or from platelets of mica and glass which are coated with metals. This pigment has a high hiding power. For applications where high transparency of the pigmented material is required, for example for agricultural films, the pigment is unsuitable. Furthermore, it has the disadvantage that the typical depth effect of interference pigments is not produced, since the high level of reflection of the light at the metal layer which forms the core means that pigment particles lying deeper within the application medium are able to make only a very small contribution to the optical appearance. The interference effect is therefore limited to the layers located on the metal layer.